2nd mobile communication refers to transmission and reception of voice through mobile communication, which includes CMDA, GSM, and the like. GPRS, advancing from the GSM, has been proposed to provide a packet switched data service based on the GSM system.
3rd generation mobile communication allows for transmission and reception of image and data, as well as voice, and 3GPP (Third Generation Partnership Project) has developed a mobile communication system (IMT-2000) technique and adopts WCDMA as a radio access technology (RAT). The combination of the IMT-2000 technique and the radio access technology (RAT), e.g., WCDMA, is called a UMTS (Universal Mobile Telecommunication System). UTRAN is an acronym of UMTS Terrestrial Radio Access Network.
3rd generation mobile communication is evolving to 4th mobile communication.
The 4th mobile communication technique includes a long-term evolution network (LTE) technique under standardization by 3GPP and an IEEE 802.16 technique under standardization by IEEE. The LTE uses a term of E-UTRAN (Evolved-UTRAN).
The 4th mobile communication technique has introduced OFDM (Orthogonal Frequency Division Multiplexing)/OFDMA (Orthogonal Frequency Division Multiple Access). OFDM uses a plurality of orthogonal subcarriers. OFDM uses orthogonality between IFFT (Inverse Fast Fourier Transform( ) and FFT (Fast Fourier Transform). A transmitter performs IFFT on data and then transmits the same. A receiver performs FFT on received signal to restore the original data. The transmitter uses IFFT in order to combine a plurality of subcarriers and the receiver uses corresponding FFT in order to split the multiple subcarriers.
Meanwhile, in the 3rd and 4th mobile communication system, attempts for increasing cell capacity continue to support high capacity services such as multimedia contents, streaming, and the like, and bi-directional services.
In order to increase cell capacity, there has been an approach of using a high frequency band and reducing a cell radius. The use of a cell, such as a pico cell, or the like, having a small cell radius allows for the use of a higher frequency band than that used for the existing cellular system, having the advantages that more information can be delivered. However, because a larger number of base stations must be necessarily installed in the same area, much cost incurs.
Thus, recently, a femto cell has been proposed as one of approaches of increasing cell capacity by using small cells.
Femto cell refers to provision of a small-scale radio environment by installing a base station using small power in indoor spaces such as homes, offices, and the like. The femto cell is expected to improve an indoor service available area and increase capacity to thus enhance quality of service (QoS), and also expected to completely settle the next generation mobile communication system by providing data services.
For such a femto cell, standardization is ongoing in the name of Home eNodeB by 3GPP WCDMA and LTE group, and 3GPP2 is also actively studying femto cell.
Various structures as illustrated in FIGS. 1 and 2 have been proposed in order to implement such a femto cell in the existing mobile communication network.
First, FIG. 1 illustrates an example of a network structure based on femto cells according to the related art.
As shown in FIG. 1, a macro base station (M-BS) serving a wider area and a plurality of femto base stations (f-BSs) installed based on users.
The f-BSs are connected with a femto cell network controller (FNC) through the Internet so as to be under the control of the FNC, and provide services to users.
A terminal measures signals of neighboring cells and delivers the measured signal values to its f-BS, and the f-BS recognizes and administers the presence of neighboring cells based on the received signal values. Also, the f-BSs exchange information through a direct link or an indirect link through the FNC. The f-BSs and the M-BS transmit and receive information through the FNC, an RNC (Radio Network Controller) or through an MME (Mobility Management Entity) that controls the f-BSs in a mobile communication network.
FIG. 2 illustrates a handover process in the system illustrated in FIG. 1.
With reference to FIG. 2, the terminal 10 periodically measures signals of the neighbor BSs while communicating with the serving BS 21, and transmits the measurement results including a value of the strength of the measured signal to the serving BS 21. The serving BS 21 may be the foregoing macro BS or the femto BS.
The serving BS 21 determines a target BS as a BS having a signal strength suitable for the terminal 10 to perform handover, among neighbor BSs, based on the measurement results, and then, in order to make the terminal 10 perform handover to the determined target BS, the serving BS 21 transmits a handover (HO) request message. The HO request message includes the information required for the handover, e.g., a cell ID (serving cell, target cell) and context of the terminal, the reason for performing handover, information of a movement path of the terminal (UE history information). The context of the terminal includes security, QoS, a user priority level, and the like.
Upon receiving the handover request message, the target BS 22 transmits a HO Request Confirm message to the serving BS 21, to inform that whether or not handover is allowed. Upon receiving the HO Request Confirm message, the serving BS 21 forwards data traffic of the terminal to the target BS 22.
The serving BS 21, transmits a handover (HO) command message to the terminal 10 to command the terminal 10 to perform handover to the target BS 22. Upon receiving the HO command message, the terminal 10 performs a handover (HO) execution process to access the target BS 22.
Meanwhile, the serving BS 21 and the target BS 22 may be macro BSs or femto BSs. In case of handover performed between the macro BS and the femto BS, the foregoing messages are transmitted or received among the femto BS, the MME, and the serving BS through the general Internet, and in this case, the Internet cannot guarantee no delay of the messages nor guarantee QoS such that the messages have a higher transmission priority level, causing handover to be delayed.
As mentioned above, when the terminal performs handover to the target femto BS or newly access the target femto BS, delay may occur because the related messages are transmitted and received through the Internet.